Inductive power transfer (IPT) systems are commonly used where contactless power transfer is required. For the most part IPT Systems are of relatively low power so that whether they are turned on or off has very little effect on the electricity supply network or grid. However, certain large-scale applications of IPT systems have the potential to have a significant effect on the grid. In particular, where an IPT system is used for widespread contactless/wireless charging of “plug-in” hybrid electric vehicles (PHEVs), their effect on the grid can be very substantial and techniques to power it must be introduced. This degree of control will be needed at first with PHEV's where the overnight charging load is now small but will continue to grow and then with roadway powered vehicles where the vehicles will be charged inductively using Inductive Power Transfer (IPT) from wires buried under the road surface directly into a pick-up on the car and then into its battery. This load too will probably start as a charging load but increasingly it will become a variable load that dynamically charges the vehicles while they are moving. Systems for achieving this must work with separations of 200-300 mm, at speeds up to and exceeding 100 kph, and in all weather conditions. IPT systems capable of doing this are beginning to appear.
The power required to drive a car and leave some reserve to charge the battery is of the order of 18 kW. A prospective load for 1,000,000 vehicles is 18 GW which even by International standards is equivalent to the output of a very large plant—18×1 GW nuclear reactors. Clearly if this power is required and can be switched on and off by the actions of the vehicle drivers then some degree of control is essential for the security of the power grid.
In today's Power Systems there is an increasing emphasis on ‘green’ power achieved using renewable sources with little or no carbon footprint. Such sources include wind, wave, and tidal power all of which are characterised by being very fluctuable to such extent that the power from them cannot be guaranteed even for only a few minutes into the future. But these power sources are important as they are carbon free and their use means that the grid frequency cannot be held as precisely as has now come to be expected. Electric vehicles either charging when parked or powered by IPT when moving along a road must therefore at some times be supplied by renewable energy sources such as wind power or wave power, but must be able to maintain their passage along the road at the required speed.
In another aspect when vehicles are moving along a road powered by IPT, power supplies to supply the roadway magnetic fields will be placed at regular intervals along the road, perhaps every 200 m, for example. These power supplies will produce power at higher frequencies, perhaps 20 kHz for IPT frequencies, and there are limits to the distance that this power can propagate before another power supply is needed. In a typical application the IPT system may use a current of 125-200 A at 20 kHz, the natural inductance of the elongate conductors that produce the magnetic field may well be 1 pH/m so that there will be a voltage drop of ωLI or nearly 16V per meter along the conductor. If the maximum allowable voltage is 800V then the voltage falls to zero after 50 m and series compensation capacitors are needed to maintain the magnetic field. These capacitors are expensive to buy and to install. In consequence, new power supplies must be used about every 200 m with each power supply actually being a dual supply that propagates current 100 m in each direction. The power supply must be able to drive all the vehicles on the 100 m of conductor attached to it. At a rating of 100 kW the supply can power 5 cars rated at 18 kW each or two buses rated at 50 kW each, but if 3 buses each rated at 50 kW are inductively coupled with a single power supply, the power supply cannot drive them all and if it collapses it makes the situation worse.
In consequence there are two conditions that the power supply must be able to manage without failure: a situation where power from the grid is simply not available as power from renewable sources is temporarily unavailable, and a situation where the instantaneous power required exceeds the rating of the power supply. Variations-on these conditions are many and include the situation where a large surge of power is available and vehicles may use it to charge their batteries even though it is only a temporary surge.
There are some current proposals for solving the power system problem. It has been suggested that all PHEV chargers could be fully reversible so that when the vehicle is parked up and charging overnight the electricity company can reverse the charging process and drain energy from the batteries of all the vehicles being charged to meet a temporary power demand. This is sometimes described as a “vehicle-to-grid” or V2G system. Such a power reversal would need significant communications synchronisation over the whole country and would have to be very fast acting. In recent times power systems collapses have occurred in spectacular fashion as power systems become overloaded. In the overloaded condition the frequency of the electricity supply network or grid reduces and as it reduces many of the generators will protect themselves by dropping out—typically at 47 Hz in New Zealand or 57 Hz in the USA. As generators drop out the frequency decreases more rapidly and the condition gets worse in what is commonly known as a cascading failure. If all the reversible battery chargers could be reversed the process could be propped up but such power reversal would have to occur very quickly—probably in less than 5 seconds—in the collapsing power system to save it.
For example, if there were 1,000,000 vehicles being charged at a 2 kW rate (rated on the typical home power socket) then potentially 2 GW of power is available country wide. A communications system to initiate the power reversal may take 1 ms per connection or 1000 seconds (¼ of an hour) which is far too slow. A system-wide message would be needed in the cellular system to do this, if it is possible. Finally there is the question of cost. An electric car has an expensive battery which is rated at say 25 kWH and is good for 1,000 discharge cycles, and costs about NZD$20,000. The amount of energy available is 25 kWH at a wholesale rate of say 6 cents per kWH=$1.50. But discharging the battery takes a cycle off its life, costing $20. There is no economic sense saving $1.50 at a cost of $20. In short therefore reversible battery chargers are not a viable solution with today's batteries or even batteries that are 10 times better.
To overcome the added complexity and response time limitations of communications systems, International Publication No. WO 2008/140333 entitled “Multi Power Sourced Electric Vehicle” describes an IPT power supply which measures variations in the frequency of the electricity supply network and uses this information to control the current supplied to the primary conductive path (power pad or track) of the IPT system, thus increasing or limiting the power available to a pickup/load or loads inductively coupled with the power supply via the primary conductive path dependent on whether the grid frequency increases or decreases with respect to a predefined frequency.
The disadvantage with the above approach is that the power delivered to the load is determined by the power supply which controls the current supplied to the primary conductive path. In some circumstances, it may be more desirable or even necessary to control the power delivered to the or each load from the secondary side of the system, i.e. the or each pick-up. In multiple pick-up IPT systems, for example, it is desirable to maintain a constant current in the primary conductive path so that the power delivered to each load can be controlled independently by the respective pick-up.
In typical IPT systems the power supply is rated at a level sufficient to energise all pick-up devices simultaneously. But this condition never actually occurs in many systems, so IPT systems typically operate with only a 40% load factor.